1. Field of the Invention
The present invention relates to a method of detecting a tension wire break in a concrete pole.
2. Description of the Related Art
Generally, concrete poles are widely used for supporting power distribution lines and other electric wires such as, for example, communication lines provided by Nippon Telegraph and Telephone Corporation (NNT), etc. and overhead contact lines for railroads and used for installing lighting systems, traffic signals, golf nets, antennas, and the like. This invention is particularly suitable for testing of concrete electric poles. Concrete electric poles are widely used for electric power distribution lines in numerous countries. The poles are generally 14 to 15 meters in height and are made in the shape of a tapered cylinder with an approximately 50-mm thick concrete wall. The outside diameter of the pole is typically about 20 cm at the top end, the diameter increasing to about 40 cm at the bottom end. In the middle of the concrete wall, the pole typically contains 12 to 20 tension wires that arc arranged around the circumference with equal angular spacing and run longitudinally along the entire length of the pole. Additionally, the concrete wall of the pole contains a number of rebar that are placed between tension wires and run longitudinally from the bottom end of the pole but not through the entire length. Typical diameters of the tension wires and rebar are 7 nun. The tension wires are prestressed during concrete casting and curing to supply the bending strength to the concrete pole needed to withstand up to hurricane-force winds.
During the long service life of a concrete electric pole, stress corrosion cracking (SCC) can develop in the tension wires due to water seeping through cracks in the concrete wall and becoming tapped around the wires. The SCC generally produces a tight fracture in tension wire. When a pole contains broken tension wires, its bending strength is significantly weakened and, unless replaced or strengthened, it could suddenly fail under gusty winds that may result in serious accidents. To ensure the structural integrity and safety of a pole, the aging concrete poles need to be inspected for the presence of broken tension wires using a suitable nondestructive evaluation (NDE) method.
As shown in FIGS. 1 and 2, in order to obtain a deflective strength necessary to resist a bending moment, a concrete pole has a xe2x80x9cprestressedxe2x80x9d structure for imparting a compressive stress to a concrete, in which a high-strength wire such as a piano wire is cast in a mold in a tensioned manner. FIG. 1 shows load imposed on supporting structure, including wind load 1, tensile load 2 and compressive load 3. FIG. 2 shows cross sections of a typical concrete pole at two levels, at 8 meters in height 7 and at 2.4 meters 8. Inside the pole are tension wires 4 and non-tension wires 5. If a tension wire is broken due to corrosion or the like, the concrete pole becomes reduced in deflective strength, which may lead to a breakage accident. Thus, a technique for nondestructive detection of a wire break is desired.
Even if the tension wire which is a high-tension steel is broken due to stress-corrosion cracking, the wire excluding a broken portion is held by the concrete without displacement, and also protected by an alkali contained in the concrete to thereby exhibiting no sectional phenomenon. The broken portion has only a slight gap, and is therefore not easily detected.
The break of the tension wire can be detected by means of radiography. However, in order to discover a wire break that cannot be specified as to where the break occurs in the concrete pole having a length of extending up to tens of meters, many radiographs are needed because a coverage per radiograph is limited. In addition, a site where the radiography is performed must be set up as a controlled area, so that it is substantially impossible to perform the radiography in an urban area.
Eddy current testing (ECT) or ultrasonic testing (UT) can also be used to detect conditions of the wire inside the concrete pole. However, the gap between broken ends of a wire is small, and the inspection is necessarily performed through concrete having a thickness of approximately 25 mm, thereby making it difficult to detect a wire break. Further, in both ECT and UT, a probe needs to be moved while being in close contact with the outer surface of the pole. However, this operation is difficult due to obstacles such as attached hardware and metal bands. Even in the case without obstacles, the probe is moved slowly in a longitudinal direction of the pole, thereby requiring much time for the inspection.
In view of the above, an object of the present invention is to make it possible to detect tension wire breaks in a concrete pole in a short period of time without scanning along the entire concrete pole with a detection device.
To achieve the above-mentioned object, a pulse of relatively low frequency (typically about 10 to 20 kHz) mechanical wave called a guided wave is generated in a wire, and propagated in its longitudinal direction, and signals reflected from the ends of the concrete pole and broken wires are detected. The signal reflected from a broken wire occurs earlier than the end-reflected signal. The presence, and the lengthwise position, of broken wires are determined from the signals occurring before the end signal and their arrival time. Among methods of generating a guided wave, a method using magnetostrictive sensors (MsS) (hereinafter, referred to as xe2x80x9cMsS methodxe2x80x9d) proposed by researchers at Southwest Research Institute (SwRI), makes it possible that a guided wave pulse is excited in a wire in a noncontact manner through magnetic coupling, and also that a reflected pulse is detected in a noncontact manner. U.S. Pat. Nos. 5,456,113, 5,457,994, 5,581,037, and 5,767,766 describe the MsS method, and are incorporated herein by reference.
The present invention is a special application of the above MsS methods modified for concrete pole tension wire inspections. While such testing of exposed wires has been previously effected, such testing of wires embedded in concrete has not only not been effected, but was previously thought impossible.
FIG. 3 shows a bias magnetic field coil for generating a static magnetic field in a longitudinal direction using a DC current and a probe coil installed to the vicinity of a top end (portion having only tension wires without rebars) of a concrete pole. FIG. 4 shows a system unit for driving the probe coil, a personal computer with keyboard for collecting data, and a bias magnetic field coil power supply.
If an AC pulse is applied to the probe coil, the magnetic field in the axial direction is changed, and a wave of condensation and rarefaction is generated in the axial direction of the wire due to a magnetostrictive effect to be propagated as the guided wave. In the case without a broken wire, a signal reflected only from an end portion of the concrete pole is detected, and in the case where the wire is broken, a signal reflected from a position where the wire is broken is detected. Generally, 12 to 20 tension wires are embedded inside a concrete pole, and if multiple wires are broken at the same position in the longitudinal direction, a reflected signal having a strength corresponding to the number of broken wires would be obtained. If many numbers of wires are broken, a signal reflected from the end portion would be weaker. If most of the wires are broken, the signal reflection at the end portion would substantially disappear. Accordingly, the level of the wire break could be determined based on the broken-wire signals and decrease in the end-reflected signals.
According to the method of the present invention, from a single test location on a concrete pole without requiring scanning along the length of the pole and without direct contact to the tension wires, wire breaks in all the tension wires in the concrete pole are simultaneously examined. in a short period of time. The above approach examines all tension wires simultaneously. Therefore, when broken wire signals are detected, which individual wires among them are broken can not be identified from the data. Individual broken wires could be identified by using the encircling coil probe in FIG. 3 as the transmitter of the guided waves and a U-shaped probe illustrated in FIG. 6 as a separate receiver that is scanned around the circumference of the pole for the maximum broken wire signal.